1. Field of the Invention
The present invention generally relates to an anode for a capacitor, and more particularly, to an electrolytic capacitor including an anode of a valve metal having an improved breakdown voltage.
2. Prior Art
Conventional construction of an electrolytic capacitor consists of anodizing a sintered porous valve metal structure, such as a tantalum pellet, in an electrolyte. In the case of tantalum, this forms a dielectric film of tantalum pentoxide on the exposed surfaces of the pellet. The electrolyte is typically composed of phosphoric acid, ethylene glycol or polyethylene glycol and water.
For a thorough discussion of the anodization process for a tantalum structure, reference is made to B. Melody et al., "An Improved Series of Electrolytes For Use In The Anodization Of Tantalum Capacitor Anodes", CARTS, pp. 40-50, 1992. This article is incorporated herein by reference.
For a thorough discussion of a conventional anodized tantalum pellet, reference is made to D. M. Smyth et al., "Heat-Treatment of Anodic Oxide Films on Tantalum", Journal of the Electrochemical Society, vol. 110, No. 12, pp 1264-1271, December 1963; D. M. Smyth et al., "Heat-Treatment of Anodic Oxide Films on Tantalum", Journal of the Electrochemical Society, vol. 110, No. 12, pp. 1271-1276, December 1963; D. M. Smyth et al., "The Heat-Treatment of Anodic Oxide Films on Tantalum", Journal of the Electrochemical Society, vol. 111, No. 12, pp. 1331-1336, December 1964; D. M. Smyth et al., "The Heat-Treatment of Anodic Oxide Films on Tantalum", Journal of the Electrochemical Society, vol. 113, No. 2, pp. 100-104, February 1966; and D. M. Smyth, "The Heat-Treatment of Anodic Oxide Films on Tantalum", Journal of the Electrochemical Society, vol. 113, No. 12, pp. 1271-1274, December 1966. These publications are incorporated herein by reference.
A pressed tantalum powder pellet is a porous structure. During the prior art anodization process, the tantalum pellet is continuously oxidized to a desired formation voltage by applying a current to the pellet. Because the tantalum pellet is porous, the electrolyte is able to flow into the pellet where it becomes heated during the anodization process. Since the anodization process is continuous, the heated electrolyte is unable to readily flow out of the pellet. Instead, the temperature of the electrolyte continues to increase through out the course of the process. It is believed that heated electrolyte is responsible for cracks, fissures and similar imperfections formed in the oxide coating and inside the tantalum pellet. These faults degrade the voltage at which the anode can be charged to.
What is needed is an improved method for manufacturing a valve metal anode such as of the kind typically used in an electrolytic capacitor. Particularly, it is desirable to provide tantalum anodes with oxide coatings devoid of cracks, fissures and similar imperfections and, consequently, having breakdown voltages that are superior to those known in the prior art.